I'm not sure if you're wanting to put the @referers
variable into a separate file, or if you're wanting
to put the check_url subroutine there, but just think
of a required file as if it were cut and pasted in
the exact spot where you put the "require" statement.

So, just cut whatever you want out of your main script,
put it in a separate file, and put the require "otherfile.pl"
in its place.

However, you should realize that the HTTP_REFERER
variable is useless as a security measure, since it
is set by the browser, and anyone can bypass your
check with some basic perl.